Self-igniting Otto operated combustion engines are known under the name gasoline HCCI-(homogeneous charge compression ignition) or CAI-procedure (controlled auto ignition). They provide a significant potential for a fuel saving at a simultaneously reduced hazardous raw emission compared to usual spark-igniting procedures.
Self-igniting Otto combustion engines are usually equipped with a variable valve operation and with a benzene direct injection. In order to create the CAI-operation different strategies are known. The common target of these strategies is to realize a relatively big remaining gas mass in the cylinder of the combustion engine. This hot remaining gas causes the introduction of the combustion during the compression phase.
The control or the regulation of the combustion in a self-igniting combustion engine advantageously takes place on the basis of a combustion chamber signal. Cylinder pressure sensors are here preferably used as sensors, which determine the pressure course during the combustion stroke in high resolution. Thereby for example the average pressure pmi in at least combustion chamber of the combustion engine can be provided as a measure for the mechanical work that is carried out by the combustion engine and the position of the combustion focus MFB50 (mass fraction burnt 50%) that is determined by the pressure course can be provided as a reference variable of the regulation.
Spark-igniting as well as self-igniting combustion engines on the basis of Otto engines are usually operated with fuel consisting of hydrocarbons made of fossil fuels on the basis of refined oil. Ethanol that has been produced from re-growing natural resources (plants) or another alcohol is additionally admixed to this fuel in different mixture ratios. In the USA and in Europe often a mixture of 70-85% ethanol and 15-30% benzene is used under the trademark E85. The combustion engines have to be construed in such a way that they can be operated with pure benzene as well as with mixture up to E85; this is termed with “flex-fuel-operation”. For an economical operation with a low emission of hazardous substances at a simultaneously high engine power and good starting behavior the operating parameters in the flex-fuel-operation have to be adjusted to the correspondingly present fuel mixture. A stoichiometric fuel-air-ratio is for example available at 14.7 volume parts of air per part of benzene, but when using ethanol an air ratio of 9 volume parts has to be adjusted. Low and/or slow changes of the alcohol content can be detected and considered by the engine management of the combustion engine for example with the aid of a lambda probe. But for example after refueling also quick changes with significant deviations of the composition of the fuel mixture can occur. According to the state of the art such fast changes of the composition of the fuel can be detected with the aid of an alcohol sensor. But this component increases the costs of the combustion engine.
The admixing of ethanol, in particular at a high admixing rate of 50% to 85%, has a significant influence upon the very sensitive CAI-combustion. Measures for compensating this influence are therefore strictly required.
From DE 10 2007 023 900.0 of the applicant a procedure is known for determining the composition of a fuel mixture consisting of a first fuel and a second fuel for operating a combustion engine, whereby the first and the second fuel provide different rates of combustion and/or different specific energy contents and whereby the combustion engine provides at least one pressure sensor in at least one combustion chamber, with which a temporal and/or angle synchronic pressure course is determined in the combustion chamber. Thereby it is disadvantageously that the influence of the ethanol admixing upon the evaluated standard characteristics, as the average induced pressure pmi, as a measure for the work that has been carried out by the combustion engine or as the position of the combustion focus MFB50, is not strong enough in order to enable a sufficiently accurate determination of the composition of the fuel mixture. This applies in particular for high admixing rates of more than 50% ethanol.
From DE 10 2007 060 223.7 of the applicant a procedure is known for determining the composition of a fuel mixture consisting of a first fuel and a second fuel or for determining the quality of a fuel for operating a combustion engine with at least one cylinder pressure sensor in at least one cylinder of the combustion engine for determining the pressure course during a combustion stoke and a cylinder pressure based engine regulation for regulating the load and combustion position of the combustion engine. It is thereby provided that the determination of the composition of the fuel mixture or the quality of the fuel takes place with the aid of regulating information of the cylinder pressure based engine regulation. Furthermore the application of the procedure is provided for determining the composition of a fuel mixture and/or the quality of a fuel for operating a spark-igniting and/or self-igniting combustion engine. The CAI-combustion procedure is in particular instable when operating with different fuel mixtures without additional measures at an operation of the combustion engine with an average to high partial load, thus at an average induced pressure higher 3 bar. The direct determination of the composition of the fuel mixture on the basis of the regulating information of the cylinder pressure based engine regulation is enabled only very inaccurately under these operating conditions.
It is the task of the invention to provide a procedure and a device, which enable an accurate determination of the fuel composition of a fuel mixture consisting of at least two fuels for operating a self-igniting combustion engine.